This invention relates to a heat resistant flexible aromatic polyimide foam.
Foamed products such as polystyrene foam and poly-olefin foam are well known. These foamed products are rigid and have low heat resistance.
Polyurethane foam is also well known as a flexible foam but also has low heat resistance.
U.S. Pat. No. 4,241,193 discloses dense, rigid polyimide foam material which is derived from mixtures of aromatic diamines and partial esters of a benzophenonetetracarboxylic acid.
European Patent Specification (EP) 0 437 821 B1 describes a method for producing polyimide products from a combination of an alkyl ester of a tetracarboxylic acid with at least one aromatic and/or heterocyclic aromatic primary diamine.
A polyimide foam product (trade name: SOLIMIDE) is commercially available from INSPEC Corp. This polyimide foam has a glass transition temperature of approx. 250xc2x0 C.
A copending U.S. patent application Ser. No. 09/895,513 describes a flexible heat resistant polyimide foam having an apparent density of 70 kg/m3 or less and a glass transition temperature of 300xc2x0 C. or higher.
In the study for improving the flexible heat resistant polyimide foam of the copending U.S. application from the viewpoint of industrial applicability, the present inventors have noted that it is not easy to precisely adust or control the apparent density or foaming ratio of the flexible polyimide foam.
It is an object of the invention to provide a flexible polyimide foam having a high heat resistance and an precisely ajusted apparent density or foaming ratio.
The invention resides in a flexible aromatic polyimide foam having an apparent density of 13.5 to 900 kg/m3 and a glass transition temperature (Tg) of 300xc2x0 C. or higher which is obtained by compressing a polyimide foam having an apparent density of 67.5 kg/m3 or less and the same glass transition temperature (Tg). The flexible aromatic polyimide foam of the invention preferably is in the form of sheet having a thickness of 1 to 50 mm.
The flexible aromatic polyimide foam of the invention can be prepared by a process which comprises the steps of:
mixing an aromatic tetracarboxylic acid comprising 2,3,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic dianhydride a portion of which is converted into a mono- or di-lower primary alcohol ester thereof with an aromatic polyamine containing diaminodisiloxane in an amount of 0.1 to 10 mol. %, based on the amount of total amine compounds, to give a solid of polyimide precursor;
heating the solid of polyimide precursor to a temperature of 300xc2x0 C. to 500xc2x0 C. to produce a polyimide foam;
and
compressing the polyimide foam by means of a uniaxial compression press.
By the compression procedure of the invention, the foamed polyimide article can be easily adjusted to have a desired apparent ratio, foaming ratio, or porosity.